Network Working Group M. Stiemerling
Request for Comments: 3989 J. Quittek
Category: Informational NEC
T. Taylor
Nortel
February 2005
Middlebox Communications (MIDCOM) Protocol Semantics
Status of This Memo
This memo provides information for the Internet community. It does
not specify an Internet standard of any kind. Distribution of this
memo is unlimited.
Copyright Notice
Copyright (C) The Internet Society (2005).
Abstract
This memo specifies semantics for a Middlebox Communication (MIDCOM)
protocol to be used by MIDCOM agents for interacting with middleboxes
such as firewalls and Network Address Translators (NATs). The
semantics discussion does not include any specification of a concrete
syntax or a transport protocol. However, a concrete protocol is
expected to implement the specified semantics or, more likely, a
superset of it. The MIDCOM protocol semantics is derived from the
MIDCOM requirements, from the MIDCOM framework, and from working
group decisions.
Table of Contents
1. Introduction ................................................. 3
1.1. Terminology ............................................ 4
1.2. Transaction Definition Template ........................ 6
2. Semantics Specification ...................................... 7
2.1. General Protocol Design ................................ 7
2.1.1. Protocol Transactions .......................... 8
2.1.2. Message Types .................................. 9
2.1.3. Session, Policy Rule, and Policy Rule Group .... 9
2.1.4. Atomicity ...................................... 10
2.1.5. Access Control ................................. 11
2.1.6. Middlebox Capabilities ......................... 11
2.1.7. Agent and Middlebox Identifiers ................ 12
2.1.8. Conformance .................................... 12
2.2. Session Control Transactions ........................... 13
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2.2.1. Session Establishment (SE) ..................... 13
2.2.2. Session Termination (ST) ....................... 15
2.2.3. Asynchronous Session Termination (AST) ......... 16
2.2.4. Session Termination by Interruption of
Connection ..................................... 17
2.2.5. Session State Machine .......................... 17
2.3. Policy Rule Transactions ............................... 18
2.3.1. Configuration Transactions ..................... 19
2.3.2. Establishing Policy Rules ...................... 19
2.3.3. Maintaining Policy Rules and Policy Rule Groups 20
2.3.4. Policy Events and Asynchronous Notifications ... 21
2.3.5. Address Tuples ................................. 21
2.3.6. Address Parameter Constraints .................. 23
2.3.7. Interface-specific Policy Rules ................ 25
2.3.8. Policy Reserve Rule (PRR) ...................... 26
2.3.9. Policy Enable Rule (PER) ....................... 30
2.3.10. Policy Rule Lifetime Change (RLC) .............. 36
2.3.11. Policy Rule List (PRL) ......................... 38
2.3.12. Policy Rule Status (PRS) ....................... 39
2.3.13. Asynchronous Policy Rule Event (ARE) ........... 41
2.3.14. Policy Rule State Machine ...................... 42
2.4. Policy Rule Group Transactions ......................... 43
2.4.1. Overview ....................................... 43
2.4.2. Group Lifetime Change (GLC) .................... 44
2.4.3. Group List (GL) ................................ 46
2.4.4. Group Status (GS) .............................. 47
3. Conformance Statements ....................................... 48
3.1. General Implementation Conformance ..................... 49
3.2. Middlebox Conformance .................................. 50
3.3. Agent Conformance ...................................... 50
4. Transaction Usage Examples ................................... 50
4.1. Exploring Policy Rules and Policy Rule Groups .......... 50
4.2. Enabling a SIP-Signaled Call ........................... 54
5. Compliance with MIDCOM Requirements .......................... 59
5.1. Protocol Machinery Requirements ........................ 59
5.1.1. Authorized Association ......................... 59
5.1.2. Agent Connects to Multiple Middleboxes ......... 60
5.1.3. Multiple Agents Connect to Same Middlebox ...... 60
5.1.4. Deterministic Behavior ......................... 60
5.1.5. Known and Stable State ......................... 60
5.1.6. Status Report .................................. 61
5.1.7. Unsolicited Messages (Asynchronous
Notifications).................................. 61
5.1.8. Mutual Authentication .......................... 61
5.1.9. Session Termination by Any Party ............... 62
5.1.10. Request Result ................................. 62
5.1.11. Version Interworking ........................... 62
5.1.12. Deterministic Handling of Overlapping Rules .... 62
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5.2. Protocol Semantics Requirements ........................ 63
5.2.1. Extensible Syntax and Semantics ................ 63
5.2.2. Policy Rules for Different Types of Middleboxes 63
5.2.3. Ruleset Groups ................................. 63
5.2.4. Policy Rule Lifetime Extension ................. 63
5.2.5. Robust Failure Modes ........................... 63
5.2.6. Failure Reasons ................................ 63
5.2.7. Multiple Agents Manipulating Same Policy Rule .. 64
5.2.8. Carrying Filtering Rules ....................... 64
5.2.9. Parity of Port Numbers ......................... 64
5.2.10. Consecutive Range of Port Numbers .............. 64
5.2.11. Contradicting Overlapping Policy Rules ......... 64
5.3. Security Requirements .................................. 65
5.3.1. Authentication, Confidentiality, Integrity ..... 65
5.3.2. Optional Confidentiality of Control Messages ... 65
5.3.3. Operation across Untrusted Domains ............. 65
5.3.4. Mitigate Replay Attacks ........................ 65
6. Security Considerations ...................................... 65
7. IAB Considerations on UNSAF .................................. 66
8. Acknowledgments .............................................. 67
9. References ................................................... 67
9.1. Normative References ................................... 67
9.2. Informative References ................................. 67
Authors' Addresses ............................................... 69
Full Copyright Statement ......................................... 70
1. Introduction
The MIDCOM working group has defined a framework [MDC-FRM] and a list
of requirements [MDC-REQ] for middlebox communication. The next step
toward a MIDCOM protocol is the specification of protocol semantics
that is constrained, but not completely implied, by the documents
mentioned above.
This memo suggests a semantics for the MIDCOM protocol. It is fully
compliant with the requirements listed in [MDC-REQ] and with the
working group's consensus on semantic issues.
In conformance with the working group charter, the semantics
description is targeted at packet filters and network address
translators (NATs), and it supports applications that require dynamic
configuration of these middleboxes.
The semantics is defined in terms of transactions. Two basic types
of transactions are used: request-reply transactions and asynchronous
transactions. For each transaction, the semantics is specified by
describing (1) the parameters of the transaction, (2) the processing
of request messages at the middlebox, and (3) the state transitions
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at the middlebox caused by the request transactions or indicated by
the asynchronous transactions, respectively, and (4) the reply and
notification messages sent from the middlebox to the agent in order
to inform the agent about the state change.
The semantics can be implemented by any protocol that supports these
two transaction types and that is sufficiently flexible concerning
transaction parameters. Different implementations for different
protocols might need to extend the semantics described below by
adding further transactions and/or adding further parameters to
transactions and/or splitting single transactions into a set of
transactions. Regardless of such extensions, the semantics below
provides a minimum necessary subset of what must be implemented.
The remainder of this document is structured as follows. Section 2
describes the protocol semantics. It is structured in four
subsections:
- General Protocol Issues (section 2.1)
- Session Control (section 2.2)
- Policy Rules (section 2.3)
- Policy Rule Groups (section 2.4)
Section 3 contains conformance statements for MIDCOM protocol
definitions and MIDCOM protocol implementations with respect to the
semantics defined in section 2. Section 4 gives two elaborated usage
examples. Finally, section 5 explains how the semantics meets the
MIDCOM requirements.
1.1. Terminology
The terminology in this memo follows the definitions given in the
framework [MDC-FRM] and requirements [MDC-REQ] document.
In addition, the following terms are used:
request transaction A request transaction consists of a
request message transfer from the agent to
the middlebox, processing of the message
at the middlebox, a reply message transfer
from the middlebox to the agent, and the
optional transfer of notification messages
from the middlebox to agents other than
the one requesting the transaction. A
request transaction might cause a state
transition at the middlebox.
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configuration transaction A configuration transaction is a request
transaction containing a request for state
change in the middlebox. If accepted, it
causes a state change at the middlebox.
monitoring transaction A monitoring transaction is a request
transaction containing a request for state
information from the middlebox. It does
not cause a state transition at the
middlebox.
asynchronous transaction An asynchronous transaction is not
triggered by an agent. It may occur
without any agent participating in a
session with the middlebox. Potentially,
an asynchronous transaction includes the
transfer of notification messages from the
middlebox to agents that participate in an
open session. A notification message is
sent to each agent that needs to be
notified about the asynchronous event.
The message indicates the state transition
at the middlebox.
agent-unique An agent-unique value is unique in the
context of the agent. This context
includes all MIDCOM sessions the agent
participates in. An agent-unique value is
assigned by the agent.
middlebox-unique A middlebox-unique value is unique in the
context of the middlebox. This context
includes all MIDCOM sessions the middlebox
participates in. A middlebox-unique value
is assigned by the middlebox.
policy rule In general, a policy rule is "a basic
building block of a policy-based system.
It is the binding of a set of actions to a
set of conditions -- where the conditions
are evaluated to determine whether the
actions are performed." [RFC3198]. In
the MIDCOM context the condition is a
specification of a set of packets to which
rules are applied. The set of actions
always contains just a single element per
rule, either action "reserve" or action
"enable".
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policy reserve rule A policy rule containing a reserve action.
The policy condition of this rule is
always true. The action is the
reservation of just an IP address or a
combination of an IP address and a range
of port numbers on neither side, one side,
or both sides of the middlebox, depending
on the middlebox configuration.
policy enable rule A policy rule containing an enable action.
The policy condition consists of a
descriptor of one or more unidirectional
or bidirectional packet flows, and the
policy action enables packets belonging to
this flow to traverse the middlebox. The
descriptor identifies the protocol, the
flow direction, and the source and
destination addresses, optionally with a
range of port numbers.
NAT binding The term NAT binding as used in this
document does not necessarily refer to a
NAT bind as defined in [NAT-TERM]. A NAT
binding in the MIDCOM semantics refers to
an abstraction that enables communication
between two end points through the NAT-
type middlebox. An enable action may
result in a NAT bind or a NAT session,
depending on the request and its
parameters.
1.2. Transaction Definition Template
In the following sections, the semantics of the MIDCOM protocol is
specified per transaction. A transaction specification contains the
following entries. Parameter entries, failure reason, and
notification message type are only specified if applicable.
transaction-name
A description name for this type of transaction.
transaction-type
The transaction type is either 'configuration', 'monitoring', or
'asynchronous'. See section 1.1 for a description of transaction
types.
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transaction-compliance
This entry contains either 'mandatory' or 'optional'. For details
see section 2.1.8.
request-parameters
This entry lists all parameters necessary for this request. A
description for each parameter is given.
reply-parameters (success)
This entry lists all parameters sent back from the middlebox to
the agent as positive response to the prior request. A
description for each parameter is given.
failure reason
All negative replies have two parameters: a request identifier
identifying the request on which the reply is sent and a parameter
indicating the failure reason. As these parameters are
compulsory, they are not listed in the template. But the template
contains a list of potential failure reasons that may be indicated
by the second parameter. The list is not exhaustive. A concrete
protocol specification may extend the list.
notification message type
The type of the notification message type that may be used by this
transaction.
semantics
This entry describes the actual semantics of the transaction.
Particularly, it describes the processing of the request message
by the middlebox, and middlebox state transitions caused by or
causing the transaction, respectively.
2. Semantics Specification
2.1. General Protocol Design
The semantics specification aims at a balance between proper support
of applications that require dynamic configuration of middleboxes and
simplicity of specification and implementation of the protocol.
Protocol interactions are structured into transactions. The state of
middleboxes is described by state machines. The state machines are
defined by states and state transitions. A single transaction may
cause or be caused by state transitions in more than one state
machine, but per state machine there is no more than one transition
per transaction.
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2.1.1. Protocol Transactions
State transitions are initiated either by a request message from the
agent to the middlebox or by some other event at the middlebox. In
the first case, the middlebox informs the agent by sending a reply
message on the actual state transition; in the second, the middlebox
sends an unsolicited asynchronous notification message to each agent
affected by the transaction (if it participates in an open session
with the middlebox).
Request and reply messages contain an agent-unique request identifier
that allows the agent to determine to which sent request a received
reply corresponds.
An analysis of the requirements showed that four kinds of
transactions are required:
- Configuration transactions allowing the agent to request state
transitions at the middlebox.
- Asynchronous transactions allowing the middlebox to change state
without a request by an agent.
- Monitoring transactions allowing the agent to request state
information from the middlebox.
- Convenience transactions combining a set of configuration
transactions.
Configuration transactions and asynchronous transactions provide the
basic MIDCOM protocol functionality. They are related to middlebox
state transitions, and they concern establishment and termination of
MIDCOM sessions and of policy rules.
Monitoring transactions are not related to middlebox state
transitions. They are used by agents to explore the number, status,
and properties of policy rules established at the middlebox.
Convenience transactions simplify MIDCOM sessions by combining a set
of configuration transactions into a single one. They are not
necessary for MIDCOM protocol operation.
As specified in detail in section 3, configuration transactions and
asynchronous transactions are mandatory. They must be implemented by
a compliant middlebox. All convenience transactions are optional,
and some of the monitoring transactions are optional.
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2.1.2. Message Types
The MIDCOM protocol supports three kinds of messages: request
messages, reply messages, and notification messages. For each kind,
different message types exist. In this semantics document, message
types are only defined by the list of parameters. The order of the
parameters and their encoding is left to a concrete protocol
definition. A protocol definition may also add further parameters to
a message type or combine several parameters into one, as long as the
information contained in the parameters defined in the semantics is
still present.
For request messages and positive reply messages there exists one
message type per request transaction. Each reply transaction defines
the parameter list of the request message and of the positive
(successful) reply message by using the transaction definition
template defined in section 1.2.
In case of a failed request transaction, a negative reply message is
sent from the middlebox to the agent. This message is the same for
all request transactions; it contains the request identifier
identifying the request to which the reply is sent and a parameter
indicating the failure reason.
There are three notification message types: the Session Termination
Notification (STN), the Policy Rule Event Notification (REN), and the
Group Event Notification (GEN). All of these contain a middlebox-
unique notification identifier.
STN The Session Termination Notification message additionally
contains a single parameter indicating the reason for session
termination by the middlebox.
REN The Policy Rule Event Notification message contains the
notification identifier, a policy rule identifier, and the
remaining policy lifetime.
GEN The Group Event Notification message contains the notification
identifier, a policy rule group identifier, and the remaining
policy rule group lifetime.
2.1.3. Session, Policy Rule, and Policy Rule Group
All transactions can be further grouped into transactions concerning
sessions, transactions concerning policy rules, and transactions
concerning policy rule groups. Policy rule groups can be used to
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indicate relationships between policy rules and to simplify
transactions on a set of policy rules by using a single transaction
per group instead of one per policy rule.
Sessions and policy rules at the middlebox are stateful. Their
states are independent of each other, and their state machines (one
per session and one per policy rule) can be separated. Policy rule
groups are also stateful, but the middlebox does not need to maintain
state for policy rule groups, because the semantics were chosen so
that the policy rule group state is implicitly defined by the state
of all policy rules belonging to the group (see section 2.4).
The separation of session state and policy rule state simplifies the
specification of the semantics as well as a protocol implementation.
Therefore, the semantics specification is structured accordingly and
we use two separated state machines to illustrate the semantics.
Please note that state machines of concrete protocol designs and
implementations will probably be more complex than the state machines
presented here. However, the protocol state machines are expected to
be a superset of the semantics state machines in this document.
2.1.4. Atomicity
All request transactions are atomic with respect to each other. This
means that processing of a request at the middlebox is never
interrupted by another request arriving or already queued. This
particularly applies when the middlebox concurrently receives
requests originating in different sessions. However, asynchronous
transactions may interrupt and/or terminate processing of a request
at any time.
All request transactions are atomic from the point of view of the
agent. The processing of a request does not start before the
complete request arrives at the middlebox. No intermediate state is
stable at the middlebox, and no intermediate state is reported to any
agent.
The number of transactions specified in this document is rather
small. Again, for simplicity, we reduced it to a minimal set that
still meets the requirements. A real implementation of the protocol
might require splitting some of the transactions specified below into
two or more transactions of the respective protocol. Reasons for
this might include constraints of the particular protocol or the
desire for more flexibility. In general this should not be a
problem. However, it should be considered that this might change
atomicity of the affected transactions.
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2.1.5. Access Control
Ownership determines access to policy rules and policy rule groups.
When a policy rule is created, a middlebox-unique identifier is
generated to identify it in further transactions. Beyond the
identifier, each policy rule has an owner. The owner is the
authenticated agent that established the policy rule. The middlebox
uses the owner attribute of a policy rule to control access to it;
each time an authenticated agent requests to modify an existing
policy rule, the middlebox determines the owner of the policy rule
and checks whether the requesting agent is authorized to perform
transactions on the owning agent's policy rules.
All policy rules belonging to the same policy rule group must have
the same owner. Therefore, authenticated agents have access either
to all members of a policy rule group, or to none of them.
The middlebox may be configured to allow specific authenticated
agents to access and modify policy rules with certain specific
owners. Certainly, a reasonable default configuration would let each
agent access its own policy rules. Also, it might be good to
configure an agent identity to act as administrator, allowing
modification of all policy rules owned by any agent. However, the
configuration of authorization at the middlebox is out of scope of
the MIDCOM semantics and protocol.
2.1.6. Middlebox Capabilities
For several reasons it is useful that at session establishment the
agent learns about particular capabilities of the middlebox.
Therefore, the session establishment procedure described in section
2.2.1 includes a transfer of capability information from the
middlebox to the agent. The list of covered middlebox capabilities
includes the following:
- Support of firewall function
- List of supported NAT functions, perhaps including
- address translation
- port translation
- protocol translation
- twice-NAT
- Internal IP address wildcard support
- External IP address wildcard support
- Port wildcard support
- Supported IP version(s) for internal network:
IPv4, IPv6, or both
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- Supported IP version(s) for external network:
IPv4, IPv6, or both
- List of supported optional MIDCOM protocol transactions
- Optional interface-specific policy rule support: not
supported or supported
- Policy rule persistence: persistent or non-persistent
(a rule is persistent when the middlebox can save the rule to
a non-volatile memory, e.g., a hard disk or flash memory)
- Maximum remaining lifetime of a policy rule or policy rule
group
- Idle-timeout of policy rules in the middlebox
(reserved and enabled policy rules not used by any
data traffic for the time of this idle-timeout are deleted
automatically by the middlebox; for the deletion of policy
rules by middleboxes, see section 2.3.13 about Asynchronous
Policy Rule Event).
- Maximum number of simultaneous MIDCOM sessions
The list of middlebox capabilities may be extended by a concrete
protocol specification with further information useful for the agent.
2.1.7. Agent and Middlebox Identifiers
To allow both agents and middleboxes to maintain multiple sessions,
each request message contains a parameter identifying the requesting
agent, and each reply message and each notification message contains
a parameter identifying the middlebox. These parameters are not
explicitly listed in the description of the individual transactions,
because they are common to all of them. They are not further
referenced in the individual semantics descriptions. Although, they
are not necessarily passed explicitly as parameters of the MIDCOM
protocol, they might be provided by the underlying (secure) transport
protocol being used. Agent identifiers at the middlebox are
middlebox-unique, and middlebox identifiers at the agent are agent-
unique, respectively.
2.1.8. Conformance
The MIDCOM requirements in [MDC-REQ] demand capabilities of the
MIDCOM protocol that are met by the set of transactions specified
below. However, an actual implementation of a middlebox may support
only a subset of these transactions. The set of announced supported
transactions may be different for different authenticated agents.
The middlebox informs the authenticated agent with the capability
exchange at session establishment about the transactions that the
agent is authorized to perform. Some transactions need to be offered
to every authenticated agent.
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Each transaction definition below has a conformance entry that
contains either 'mandatory' or 'optional'. A mandatory transaction
needs to be implemented by every middlebox offering MIDCOM service
and must be must be offered to each of the authenticated agents. An
optional transaction does not necessarily need to be implemented by a
middlebox; it may offer these optional transactions only to certain
authenticated agents. The middlebox may offer one, several, all, or
no optional transactions to the agents. Whether an agent is allowed
to use an optional request transaction is determined by the
middlebox's authorization procedure, which is not further specified
by this document.
2.2. Session Control Transactions
Before any transaction on policy rules or policy rule groups is
possible, a valid MIDCOM session must be established. A MIDCOM
session is an authenticated and authorized association between agent
and middlebox. Sessions are initiated by agents and can be
terminated by either the agent or the middlebox. Both agent and
middlebox may participate in several sessions (with different
entities) at the same time. To distinguish different sessions, each
party uses local session identifiers.
All transactions are transmitted within this MIDCOM session.
Session control is supported by three transactions:
- Session Establishment (SE)
- Session Termination (ST)
- Asynchronous Session Termination (AST)
The first two are configuration transactions initiated by the agent,
and the last one is an asynchronous transaction initiated by the
middlebox.
2.2.1. Session Establishment (SE)
transaction-name: session establishment
transaction-type: configuration
transaction-compliance: mandatory
request-parameters:
- request identifier: An agent-unique identifier for matching
corresponding request and reply at the agent.
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- version: The version of the MIDCOM protocol.
- middlebox authentication challenge (mc): An authentication
challenge token for authentication of the middlebox. As seen
below, this is present only in the first iteration of the
request.
- agent authentication (aa): An authentication token
authenticating the agent to the middlebox. As seen below, this
is updated in the second iteration of the request with material
responding to the middlebox challenge.
reply-parameters (success):
- request identifier: An identifier matching the identifier
request.
- middlebox authentication (ma): An authentication token
authenticating the middlebox to the agent.
- agent challenge token (ac): An authentication challenge token
for the agent authentication.
- middlebox capabilities: A list describing the middlebox's
capabilities. See section 2.1.6 for the list of middlebox
capabilities.
failure reason:
- authentication failed
- no authorization
- protocol version of agent and middlebox do not match
- lack of resources
semantics:
This session establishment transaction is used to establish a
MIDCOM session. For mutual authentication of both parties two
subsequent session establishment transactions are required as
shown in Figure 1.
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agent middlebox
| session establishment request |
| (with middlebox challenge mc) | CLOSED
|-------------------------------------------->|
| |
| successful reply (with middlebox |
| authentication ma and agent challenge ac) |
||
| |
| successful reply |
|
RFC 3989 MIDCOM Protocol Semantics February 2005
- request identifier: An agent-unique identifier for matching
corresponding request and reply at the agent.
reply-parameters (success only):
- request identifier: An identifier matching the identifier of the
request.
semantics:
This transaction is used to close the MIDCOM session on behalf of
the agent. After session termination, the middlebox keeps all
established policy rules until their lifetime expires or until an
event occurs that causes the middlebox to terminate them.
The middlebox always generates a successful reply. After sending
the reply, the middlebox will not send any further messages to the
agent within the current session. It also will not process any
further request within this session that it received while
processing the session termination request, or that it receives
later.
2.2.3. Asynchronous Session Termination (AST)
transaction-name: asynchronous session termination
transaction-type: asynchronous
transaction-compliance: mandatory
notification message type: Session Termination Notification (STN)
reply-parameters (success only):
- termination reason: The reason why the session is terminated.
semantics:
The middlebox may decide to terminate a MIDCOM session at any
time. Before terminating the actual session the middlebox
generates a STN message and sends it to the agent. After sending
the notification, the middlebox will not process any further
request by the agent, even if it is already queued at the
middlebox.
After session termination, the middlebox keeps all established
policy rules until their lifetime expires or until an event occurs
for which the middlebox terminates them.
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Unlike in other asynchronous transactions, no more than one
notification is sent, because there is only one agent affected by
the transaction.
2.2.4. Session Termination by Interruption of Connection
If a MIDCOM session is based on an underlying network connection, the
session can also be terminated by an interruption of this connection.
If the middlebox detects this, it immediately terminates the session.
The effect on established policy rules is the same as for the
Asynchronous Session Termination.
2.2.5. Session State Machine
A state machine illustrating the semantics of the session
transactions is shown in Figure 2. The transaction abbreviations
used can be found in the headings of the particular transaction
section.
All sessions start in state CLOSED. If mutual authentication is
already provided by other means, a successful SE transaction can
cause a state transition to state OPEN. Otherwise, it causes a
transition to state NOAUTH. From this state a failed second SE
transaction returns to state CLOSED. A successful SE transaction
causes a transition to state OPEN. At any time, an AST transaction
or a connection failure may occur, causing a transition to state
CLOSED. A successful ST transaction from either NOAUTH or OPEN also
causes a return to CLOSED. The parameters of the transactions are
explained in Figure 2; the value mc=0 represents an empty middlebox
challenge.
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mc = middlebox challenge
SE/failure ma = middlebox authentication
+-------+ ac = agent challenge
| v aa = agent authentication
+----------+
| CLOSED |----------------+
+----------+ | SE(mc!=0)/
| ^ ^ | success(ma,ac)
SE(mc=0, | | | AST |
aa=OK)/ | | | SE/failure v
success | | | ST/success +----------+
| | +------------| NOAUTH |
| | +----------+
| | AST | SE(mc=0,
v | ST/success | aa=OK)/
+----------+ | success
| OPEN |
RFC 3989 MIDCOM Protocol Semantics February 2005
2.3.1. Configuration Transactions
Policy Rule transactions PER and RLC constitute the core of the
MIDCOM protocol. Both are mandatory, and they serve for
- configuring NAT bindings (PER)
- configuring firewall pinholes (PER)
- extending the lifetime of established policy rules (RLC)
- deleting policy rules (RLC)
Some cases require knowing in advance which IP address (and port
number) would be chosen by NAT in a PER transaction. This
information is required before sufficient information for performing
a complete PER transaction is available (see example in section 4.2).
For supporting such cases, the core transactions are extended by the
Policy Reserve Rule (PRR) transaction serving for
- reserving addresses and port numbers at NATs (PRR)
2.3.2. Establishing Policy Rules
Both PRR and PER establish a policy rule. The action within the rule
is 'reserve' if set by PRR and 'enable' if set by PER.
The Policy Reserve Rule (PRR) transaction is used to establish an
address reservation on neither side, one side, or both sides of the
middlebox, depending on the middlebox configuration. The transaction
returns the reserved IP addresses and the optional ranges of port
numbers to the agent. No address binding or pinhole configuration is
performed at the middlebox. Packet processing at the middlebox
remains unchanged.
On pure firewalls, the PRR transaction is successfully processed
without any reservation, but the state transition of the MIDCOM
protocol engine is exactly the same as on NATs.
On a traditional NAT (see [NAT-TRAD]), only an external address is
reserved; on a twice-NAT, an internal and an external address are
reserved. The reservation at a NAT is for required resources, such
as IP addresses and port numbers, for future use. How the
reservation is exactly done depends on the implementation of the NAT.
In both cases the reservation concerns either an IP address only or a
combination of an IP address with a range of port numbers.
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The Policy Enable Rule (PER) transaction is used to establish a
policy rule that affects packet processing at the middlebox.
Depending on its input parameters, it may make use of the reservation
established by a PRR transaction or create a new rule from scratch.
On a NAT, the enable action is interpreted as a bind action
establishing bindings between internal and external addresses. At a
firewall, the enable action is interpreted as one or more allow
actions configuring pinholes. The number of allow actions depends on
the parameters of the request and the implementation of the firewall.
On a combined NAT/firewall, the enable action is interpreted as a
combination of bind and allow actions.
The PRR transaction and the PER transaction are described in more
detail in sections 2.3.8 and 2.3.9 below.
2.3.3. Maintaining Policy Rules and Policy Rule Groups
Each policy rule has a middlebox-unique identifier.
Each policy rule has an owner. Access control to the policy rule is
based on ownership (see section 2.1.5). Ownership of a policy rule
does not change during lifetime of the policy rule.
Each policy rule has an individual lifetime. If the policy rule
lifetime expires, the policy rule will be terminated at the
middlebox. Typically, the middlebox indicates termination of a
policy rule by an ARE transaction. A policy rule lifetime change
(RLC) transaction may extend the lifetime of the policy rule up to
the limit specified by the middlebox at session setup. Also an RLC
transaction may be used for shortening a policy rule's lifetime or
deleting a policy rule by requesting a lifetime of zero. (Please
note that policy rule lifetimes may also be modified by the group
lifetime change (GLC) transaction.)
Each policy rule is a member of exactly one policy rule group. Group
membership does not change during the lifetime of a policy rule.
Selecting the group is part of the transaction establishing the
policy rule. This transaction implicitly creates a new group if the
agent does not specify one. The new group identifier is chosen by
the middlebox. New members are added to an existing group if the
agent's request designates one. A group only exists as long as it
has member policy rules. As soon as all policies belonging to the
group have reached the ends of their lifetimes, the group does not
exist anymore.
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Agents can explore the properties and status of all policy rules they
are allowed to access by using the Policy Rule Status (PRS)
transaction.
2.3.4. Policy Events and Asynchronous Notifications
If a policy rule changes its state or if its remaining lifetime is
changed in ways other than being decreased by time, then all agents
that can access this policy rule and that participate in an open
session with the middlebox are notified by the middlebox. If the
state or lifetime change was requested explicitly by a request
message, then the middlebox notifies the requesting agent by
returning the corresponding reply. All other agents that can access
the policy are notified by a Policy Rule Event Notification (REN)
message.
Note that a middlebox can serve multiple agents at the same time in
different parallel sessions. Between these agents, the sets of
policy rules that can be accessed by them may overlap. For example,
there might be an agent that authenticates as administrator and that
can access all policies of all agents. Or there could be a backup
agent running a session in parallel to a main agent and
authenticating itself as the same entity as the main agent.
In case of a PER, PRR, or RLC transaction, the requesting agent
receives a PER, PRR, or RLC reply, respectively. To all other agents
that can access the created, modified, or terminated policy rule (and
that participate in an open session with the middlebox) the middlebox
sends an REN message carrying the policy rule identifier (PID) and
the remaining lifetime of the policy rule.
In case of a rule termination by lifetime truncation or other events
not triggered by an agent, then the middlebox sends an REN message to
each agent that can access the particular policy rule and that
participates in an open session with the middlebox. This ensures
that an agent always knows the most recent state of all policy rules
it can access.
2.3.5. Address Tuples
Request and reply messages of the PRR, PER, and PRS transactions
contain address specifications for IP and transport addresses. These
parameters include
- IP version
- IP address
- IP address prefix length
- transport protocol
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- port number
- port parity
- port range
Additionally, the request message of PER and the reply message of PRS
contain a direction of flow parameter. This direction of flow
parameter indicates for UDP and IP the direction of packets
traversing the middlebox. For 'inbound', the UDP packets are
traversing from outside to inside; for 'outbound', from inside to the
outside. In both cases, the packets can traverse the middelbox only
uni-directionally. A bi-directional flow is enabled through 'bi-
directional' as direction of flow parameter. For TCP, the packet
flow is always bi-directional, but the direction of the flow
parameter is defined as
- inbound: bi-directional TCP packet flow. First packet, with TCP
SYN flag set and ACK flag not set, must arrive at the middlebox
at the outside interface.
- outbound: bi-directional TCP packet flow. First packet, with
TCP SYN flag set and ACK flag not set, must arrive at the
middlebox at the inside interface.
- bi-directional: bi-directional TCP packet flow. First packet,
with TCP SYN flag set and ACK flag not set, may arrive at inside
or outside interface.
We refer to the set of these parameters as an address tuple. An
address tuple specifies either a communication endpoint at an
internal or external device or allocated addresses at the middlebox.
In this document, we distinguish four kinds of address tuples, as
shown in Figure 3.
+----------+ +----------+
| internal | A0 A1 +-----------+ A2 A3 | external |
| endpoint +----------+ middlebox +----------+ endpoint |
+----------+ +-----------+ +----------+
Figure 3: Address tuples A0 - A3
- A0 -- internal endpoint: Address tuple A0 specifies a
communication endpoint of a device within -- with respect to the
middlebox -- the internal network.
- A1 -- middlebox inside address: Address tuple A1 specifies a
virtual communication endpoint at the middlebox within the
internal network. A1 is the destination address for packets
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passing from the internal endpoint to the middlebox and is the
source for packets passing from the middlebox to the internal
endpoint.
- A2 -- middlebox outside address: Address tuple A2 specifies a
virtual communication endpoint at the middlebox within the
external network. A2 is the destination address for packets
passing from the external endpoint to the middlebox and is the
source for packets passing from the middlebox to the external
endpoint.
- A3 -- external endpoint: Address tuple A3 specifies a
communication endpoint of a device within -- with respect to the
middlebox -- the external network.
For a firewall, the inside and outside endpoints are identical to the
corresponding external or internal endpoints, respectively. In this
case the installed policy rule sets the same value in A2 as in A0
(A0=A2) and sets the same value in A1 as in A3 (A1=A3).
For a traditional NAT, A2 is given a value different from that of A0,
but the NAT binds them. As for the firewall, it is also as it is at
a traditional NAT: A1 has the same value as A3.
For a twice-NAT, there are two bindings of address tuples: A1 and A2
are both assigned values by the NAT. The middlebox outside address
A2 is bound to the internal endpoint A0, and the middlebox inside
address A1 is bound to the external endpoint A3.
2.3.6. Address Parameter Constraints
For transaction parameters belonging to an address tuple, some
constraints exist that are common for all messages using them.
Therefore, these constraints are summarized in the following and are
not repeated again when describing the parameters in the transaction
descriptions are presented.
The MIDCOM semantics defined in this document specifies the handling
of IPv4 and IPv6 as network protocols, and of TCP and UDP (over IPv4
and IPv6) as transport protocols. The handling of any other
transport protocol, e.g., SCTP, is not defined within the semantics
but may be supported by concrete protocol specifications.
The IP version parameter has either the value 'IPv4' or 'IPv6'. In a
policy rule, the value of the IP version parameter must be the same
for address tuples A0 and A1, and for A2 and A3.
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The value of the IP address parameter must conform with the specified
IP version.
The IP address of an address tuple may be wildcarded. Whether IP
address wildcarding is allowed or in which range it is allowed
depends on the local policy of the middlebox; see also section 6,
"Security Considerations". Wildcarding is specified by the IP
address prefix length parameter of an address tuple. In line with
the common use of a prefix length, this parameter indicates the
number of high significant bits of the IP address that are fixed,
while the remaining low significant bits of the IP address are
wildcarded.
The value of the transport protocol parameter can be either 'TCP',
'UDP', or 'ANY'. If the transport protocol parameter has the value
'ANY', only IP headers are considered for packet handling in the
middlebox -- i.e., the transport header is not considered. The
values of the parameters port number, port range, and port parity are
irrelevant if the protocol parameter is 'ANY'. In a policy rule, the
value of the transport protocol parameter must be the same for all
address tuples A0, A1, A2, and A3.
The value of the port number parameter is either zero or a positive
integer. A positive integer specifies a concrete UDP or TCP port
number. The value zero specifies port wildcarding for the protocol
specified by the transport protocol parameter. If the port number
parameter has the value zero, then the value of the port range
parameter is irrelevant. Depending on the value of the transport
protocol parameter, this parameter may truly refer to ports or may
refer to an equivalent concept.
The port parity parameter is differently used in the context of
policy reserve rules (PRR) and policy enable rules (PER). In the
context of a PRR, the value of the parameter may be 'odd', 'even', or
'any'. It specifies the parity of the first (lowest) reserved port
number.
In the context of a PER, the port parity parameter indicates to the
middlebox whether port numbers allocated at the middlebox should have
the same parity as the corresponding internal or external port
numbers, respectively. In this context, the parameter has the value
'same' or 'any'. If the value is 'same', then the parity of the port
number of A0 must be the same as the parity of the port number of A2,
and the parity of the port number of A1 must be the same as the
parity of the port number of A3. If the port parity parameter has
the value 'any', then there are no constraints on the parity of any
port number.
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The port range parameter specifies a number of consecutive port
numbers. Its value is a positive integer. Like the port number
parameter, this parameter defines a set of consecutive port numbers
starting with the port number specified by the port number parameter
as the lowest port number and having as many elements as specified by
the port range parameter. A value of 1 specifies a single port
number. The port range parameter must have the same value for each
address tuple A0, A1, A2, and A3.
A single policy rule P containing a port range value greater than one
is equivalent to a set of policy rules containing a number n of
policies P_1, P_2, ..., P_n where n equals the value of the port
range parameter. Each policy rule P_1, P_2, ..., P_n has a port
range parameter value of 1. Policy rule P_1 contains a set of
address tuples A0_1, A1_1, A2_1, and A3_1, each of which contains the
first port number of the respective address tuples in P; policy rule
P_2 contains a set of address tuples A0_2, A1_2, A2_2, and A3_2, each
of which contains the second port number of the respective address
tuples in P; and so on.
2.3.7. Interface-specific Policy Rules
Usually agents request policy rules with the knowledge of A0 and A3
only, i.e., the address tuples (see section 2.3.5). But in very
special cases, agents may need to select the interfaces to which the
requested policy rule is bound. Generally, the middlebox is careful
about choosing the right interfaces when reserving or enabling a
policy rule, as it has the overall knowledge about its configuration.
For agents that want to select the interfaces, optional parameters
are included in the Policy Reserve Rule (PRR) and Policy Enable Rule
(PER) transactions. These parameters are called
- inside interface: The selected interface at the inside of the
middlebox -- i.e., in the private or protected address realm.
- outside interface: The selected interface at the outside of the
middlebox -- i.e., in the public address realm.
The Policy Rule Status (PRS) transactions include these optional
parameters in its replies when they are supported.
Agents can learn at session startup whether interface-specific policy
rules are supported by the middlebox, by checking the middlebox
capabilities (see section 2.1.6).
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2.3.8. Policy Reserve Rule (PRR)
transaction-name: policy reserve rule
transaction-type: configuration
transaction-compliance: mandatory
request-parameters:
- request identifier: An agent-unique identifier for matching
corresponding request and reply at the agent.
- group identifier: A reference to the group of which the policy
reserve rule should be a member. As indicated in section 2.3.3,
if this value is not supplied, the middlebox assigns a new group
for this policy reserve rule.
- service: The requested NAT service of the middlebox. Allowed
values are 'traditional' or 'twice'.
- internal IP version: Requested IP version at the inside of the
middlebox; see section 2.3.5.
- internal IP address: The IP address of the internal
communication endpoint (A0 in Figure 3); see section 2.3.5.
- internal port number: The port number of the internal
communication endpoint (A0 in Figure 3); see section 2.3.5.
- inside interface (optional): Interface at the inside of the
middlebox; see section 2.3.7.
- external IP version: Requested IP version at the outside of the
middlebox; see section 2.3.5.
- outside interface (optional): Interface at the outside of the
middlebox; see Section 2.3.7.
- transport protocol: See section 2.3.5.
- port range: The number of consecutive port numbers to be
reserved; see section 2.3.5.
- port parity: The requested parity of the first (lowest) port
number to be reserved; allowed values for this parameter are
'odd', 'even', and 'any'. See also section 2.3.5.
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- policy rule lifetime: A lifetime proposal to the middlebox for
the requested policy rule.
reply-parameters (success):
- request identifier: An identifier matching the identifier of the
request.
- policy rule identifier: A middlebox-unique policy rule
identifier. It is assigned by the middlebox and used as policy
rule handle in further policy rule transactions, particularly to
refer to the policy reserve rule in a subsequent PER
transaction.
- group identifier: A reference to the group of which the policy
reserve rule is a member.
- reserved inside IP address: The reserved IPv4 or IPv6 address on
the internal side of the middlebox. For an outbound flow, this
will be the destination to which the internal endpoint sends its
packets (A1 in Figure 3). For an inbound flow, it will be the
apparent source address of the packets as forwarded to the
internal endpoint (A0 in Figure 3). The middlebox reserves and
reports an internal address only in the case where twice-NAT is
in effect. Otherwise, an empty value for the addresses
indicates that no internal reservation was made. See also
Section 2.3.5.
- reserved inside port number: See section 2.3.5.
- reserved outside IP address: The reserved IPv4 or IPv6 address
on the external side of the middlebox. For an inbound flow,
this will be the destination to which the external endpoint
sends its packets (A2 in Figure 4). For an outbound flow, it
will be the apparent source address of the packets as forwarded
to the external endpoint (A3 in Figure 3). If the middlebox is
configured as a pure firewall, an empty value for the addresses
indicates that no external reservation was made. See also
section 2.3.5.
- reserved outside port number: See section 2.3.5.
- policy rule lifetime: The policy rule lifetime granted by the
middlebox, after which the reservation will be revoked if it has
not been replaced already by a policy enable rule in a PER
transaction.
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failure reason:
- agent not authorized for this transaction
- agent not authorized to add members to this group
- lack of IP addresses
- lack of port numbers
- lack of resources
- specified inside/outside interface does not exist
- specified inside/outside interface not available for specified
service
notification message type: Policy Rule Event Notification (REN)
semantics:
The agent can use this transaction type to reserve an IP address
or a combination of IP address, transport type, port number, and
port range at neither side, one side, or both sides of the
middlebox as required to support the enabling of a flow.
Typically the PRR will be used in scenarios where it is required
to perform such a reservation before sufficient parameters for a
complete policy enable rule transaction are available. See
section 4.2 for an example.
When receiving the request, the middlebox determines how many
address (and port) reservations are required based on its
configuration. If it provides only packet filter services, it
does not perform any reservation and returns empty values for the
reserved inside and outside IP addresses and port numbers. If it
is configured for twice-NAT, it reserves both inside and outside
IP addresses (and an optional range of port numbers) and returns
them. Otherwise, it reserves and returns an outside IP address
(and an optional range of port numbers) and returns empty values
for the reserved inside address and port range.
The A0 parameter (inside IP address version, inside IP address,
and inside port number) can be used by the middlebox to determine
the correct NAT mapping and thus A2 if necessary. Once a PRR
transaction has reserved an outside address (A2) for an internal
end point (A0) at the middlebox, the middlebox must ensure that
this reserved A2 is available in any subsequent PER and PRR
transaction.
For middleboxes supporting interface-specific policy rules, as
defined in section 2.3.7, the optional inside and outside
interface parameters must both be included in the request, or
neither of them should be included. In the presence of these
parameters, the middlebox uses the outside interface parameter to
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select the interface at which the outside address tuple (outside
IP address and port number) is reserved, and the inside interface
parameter to select the interface at which the inside address
tuple (inside IP address and port number) is reserved. Without
the presence of these parameters, the middlebox selects the
particular interfaces based on its internal configuration.
If there is a lack of resources, such as available IP addresses,
port numbers, or storage for further policy rules, then the
reservation fails, and an appropriate failure reply is generated.
If a non-existing policy rule group was specified, or if an
existing policy rule group was specified that is not owned by the
requesting agent, then no new policy rule is established, and an
appropriate failure reply is generated.
In case of success, this transaction creates a new policy reserve
rule. If an already existing policy rule group is specified, then
the new policy rule becomes a member of it. If no policy group is
specified, a new group is created with the new policy rule as its
only member. The middlebox generates a middlebox-unique
identifier for the new policy rule. The owner of the new policy
rule is the authenticated agent that sent the request. The
middlebox chooses a lifetime value that is greater than zero and
less than or equal to the minimum of the requested value and the
maximum lifetime specified by the middlebox at session startup,
i.e.,
0 <= lt_granted <= MINIMUM(lt_requested, lt_maximum)
where
- lt_granted is the lifetime actually granted by the middlebox
- lt_requested is the lifetime the agent requested
- lt_maximum is the maximum lifetime specified at session
setup
A middlebox with NAT capability always reserves a middlebox
external address tuple (A2) in response to a PRR request. In the
special case of a combined twice-NAT/NAT middlebox, the agent can
request only NAT service or twice-NAT service by choosing the
service parameter 'traditional' or 'twice', respectively. An
agent that does not have any preference chooses 'twice'. The
'traditional' value should only be used in order to select
traditional NAT service at middleboxes offering both traditional
NAT and twice NAT. In the 'twice' case, the combined twice-
NAT/NAT middlebox reserves A2 and A1; the 'traditional' case
results in a reservation of A2 only. An agent
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must always use the PRR transaction for choosing NAT only or
twice-NAT service in the special case of a combined twice-NAT/NAT
middlebox. A firewall middlebox ignores this parameter.
If the protocol identifier is 'ANY', then the middlebox reserves
available inside and/or outside IP address(es) only. The reserved
address(es) are returned to the agent. In this case, the
request-parameters "port range" and "port parity" as well as
reply-parameters "inside port number" and "outside port number",
are irrelevant.
If the protocol identifier is 'UDP' or 'TCP', then a combination
of an IP address and a consecutive sequence of port numbers,
starting with the specified parity, is reserved, on neither side,
one side, or both sides of the middlebox, as appropriate. The IP
address(es) and the first (lowest) reserved port number(s) of the
consecutive sequence are returned to the agent. (This also
applies to other protocols supporting ports or the equivalent.)
After a new policy reserve rule is successfully established and
the reply message has been sent to the requesting agent, the
middlebox checks whether there are other authenticated agents
participating in open sessions, which can access the new policy
rule. If the middlebox finds one or more of these agents, then it
sends a REN message reporting the new policy rule to each of them.
MIDCOM agents use the policy enable rule (PER) transaction to enable
policy reserve rules that have been established beforehand by a
policy reserve rule (PRR) transaction. See also section 2.3.2.
2.3.9. Policy Enable Rule (PER)
transaction-name: policy enable rule
transaction-type: configuration
transaction-compliance: mandatory
request-parameters:
- request identifier: An agent-unique identifier for matching
corresponding request and reply at the agent.
- policy reserve rule identifier: A reference to an already
existing policy reserve rule created by a PRR transaction. The
reference may be empty, in which case the middlebox must assign
any necessary addresses and port numbers within this PER
transaction. If it is not empty, then the following request
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parameters are irrelevant: group identifier, transport protocol,
port range, port parity, internal IP version, external IP
version.
- group identifier: A reference to the group of which the policy
enable rule should be a member. As indicated in section 2.3.3,
if this value is not supplied, the middlebox assigns a new group
for this policy reserve rule.
- transport protocol: See section 2.3.5.
- port range: The number of consecutive port numbers to be
reserved; see section 2.3.5.
- port parity: The requested parity of the port number(s) to be
mapped. Allowed values of this parameter are 'same' and 'any'.
See also section 2.3.5.
- direction of flow: This parameter specifies the direction of
enabled communication, either 'inbound', 'outbound', or 'bi-
directional'.
- internal IP version: Requested IP version at the inside of the
middlebox; see section 2.3.5.
- internal IP address: The IP address of the internal
communication endpoint (A0 in Figure 3); see section 2.3.5.
- internal port number: The port number of the internal
communication endpoint (A0 in Figure 3); see section 2.3.5.
- inside interface (optional): Interface at the inside of the
middlebox; see section 2.3.7.
- external IP version: Requested IP version at the outside of the
middlebox; see section 2.3.5.
- external IP address: The IP address of the external
communication endpoint (A3 in Figure 3); see section 2.3.5.
- external port number: The port number of the external
communication endpoint (A3 in Figure 4), see section 2.3.5.
- outside interface (optional): Interface at the outside of the
middlebox; see section 2.3.7.
- policy rule lifetime: A lifetime proposal to the middlebox for
the requested policy rule.
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reply-parameters (success):
- request identifier: An identifier matching the identifier of the
request.
- policy rule identifier: A middlebox-unique policy rule
identifier. It is assigned by the middlebox and used as policy
rule handle in further policy rule transactions. If a policy
reserve rule identifier was provided in the request, then the
returned policy rule identifier has the same value.
- group identifier: A reference to the group of which the policy
enable rule is a member. If a policy reserve rule identifier
was provided in the request, then this parameter identifies the
group of which the policy reserve rule was a member.
- inside IP address: The IP address provided at the inside of the
middlebox (A1 in Figure 3). In case of a twice-NAT, this
parameter will be an internal IP address reserved at the inside
of the middlebox. In all other cases, this reply-parameter will
be identical with the external IP address passed with the
request. If the policy reserve rule identifier parameter was
supplied in the request and the respective PRR transaction
reserved an inside IP address, then the inside IP address
provided in the PER response will be the identical value to that
returned by the response to the PRR request. See also section
2.3.5.
- inside port number: The internal port number provided at the
inside of the middlebox (A1 in Figure 3); see also section
2.3.5.
- outside IP address: The external IP address provided at the
outside of the middlebox (A2 in Figure 4). In case of a pure
firewall, this parameter will be identical with the internal IP
address passed with the request. In all other cases, this
reply-parameter will be an external IP address reserved at the
outside of the middlebox. See also section 2.3.5.
- outside port number: The external port number provided at the
outside of the NAT (A2 in Figure 3); see section 2.3.5..
- policy rule lifetime: The policy rule lifetime granted by the
middlebox.
failure reason:
- agent not authorized for this transaction
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- agent not authorized to add members to this group
- no such policy reserve rule
- agent not authorized to replace this policy reserve rule
- conflict with already existing policy rule (e.g., the same
internal address-port is being mapped to different outside
address-port pairs)
- lack of IP addresses
- lack of port numbers
- lack of resources
- no internal IP wildcarding allowed
- no external IP wildcarding allowed
- specified inside/outside interface does not exist
- specified inside/outside interface not available for specified
service
- reserved A0 to requested A0 mismatch
notification message type: Policy Rule Event Notification (REN)
semantics:
This transaction can be used by an agent to enable communication
between an internal endpoint and an external endpoint
independently of the type of middlebox (NAT, NAPT, firewall, NAT-
PT, combined devices), for unidirectional or bi-directional
traffic.
The agent sends an enable request specifying the endpoints
(optionally including wildcards) and the direction of
communication (inbound, outbound, bi-directional). The
communication endpoints are displayed in Figure 3. The basic
operation of the PER transaction can be described by
1. the agent sending A0 and A3 to the middlebox,
2. the middlebox reserving A1 and A2 or using A1 and A2 from a
previous PRR transaction,
3. the middlebox enabling packet transfer between A0 and A3 by
binding A0-A2 and A1-A3 and/or by opening the corresponding
pinholes, both according to the specified direction, and
4. the middlebox returning A1 and A2 to the agent.
In case of a pure packet filtering firewall, the returned address
tuples are the same as those in the request: A2=A0 and A1=A3.
Each partner uses the other's real address. In case of a
traditional NAT, the internal endpoint may use the real address of
the external endpoint (A1=A3), but the external endpoint uses an
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address tuple provided by the NAT (A2!=A0). In case of a twice-
NAT device, both endpoints use address tuples provided by the NAT
for addressing their communication partner (A3!=A1 and A2!=A0).
If a firewall is combined with a NAT or a twice-NAT, the replied
address tuples will be the same as for pure traditional NAT or
twice-NAT, respectively, but the middlebox will configure its
packet filter in addition to the performed NAT bindings. In case
of a firewall combined with a traditional NAT, the policy rule may
imply more than one enable action for the firewall configuration,
as incoming and outgoing packets may use different source-
destination pairs.
For middleboxes supporting interface specific policy rules, as
defined in Section 2.3.7, the optional inside and outside
interface parameters must both be included in the request, or
neither of them should be included. In the presence of these
parameters, the middlebox uses the outside interface parameter to
select the interface at which the outside address tuple (outside
IP address and port number) is bound, and the inside interface
parameter to select the interface at which the inside address
tuple (inside IP address and port number) is bound. Without the
presence of these parameters, the middlebox selects the particular
interfaces based on its internal configuration.
Checking the Policy Reservation Rule Identifier
If the parameter specifying the policy reservation rule
identifier is not empty, then the middlebox checks whether the
referenced policy rule exists, whether the agent is authorized
to replace this policy rule, and whether this policy rule is a
policy reserve rule.
In case of success, this transaction creates a new policy
enable rule. If a policy reserve rule was referenced, then the
policy reserve rule is terminated without an explicit
notification sent to the agent (other than the successful PER
reply).
The PRR transaction sets the internal endpoint A0 during the
reservation process. In the process of creating a new policy
enable rule, the middlebox may check whether the requested A0
is equal to the reserved A0. The middlebox may reject a PER
request with a requested A0 not equal to the reserved A0 and
must then send an appropriate failure message. Alternatively,
the middlebox may change A0 due to the PER request.
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The middlebox generates a middlebox-unique identifier for the
new policy rule. If a policy reserve rule was referenced, then
the identifier of the policy reserve rule is reused.
The owner of the new policy rule is the authenticated agent
that sent the request.
Checking the Policy Rule Group Identifier
If no policy reserve rule was specified, then the policy rule
group parameter is checked. If a non-existing policy rule
group is specified, or if an existing policy rule group is
specified that is not owned by the requesting agent, then no
new policy rule is established, and an appropriate failure
reply is generated.
If an already existing policy rule group is specified, then the
new policy rule becomes a member. If no policy group is
specified, then a new group is created with the new policy rule
as its only member.
If the transport protocol parameter value is 'ANY', then the
middlebox enables communication between the specified external IP
address and the specified internal IP address. The addresses to
be used by the communication partners to address each other are
returned to the agent as inside IP address and outside IP address.
If the reservation identifier is not empty and if the reservation
used the same transport protocol type, then the reserved IP
addresses are used.
For the transport protocol parameter values 'UDP' and 'TCP', the
middlebox acts analogously as for 'ANY' but also maps ranges of
port numbers, keeping the port parity, if requested.
The configuration of the middlebox may fail because of lack of
resources, such as available IP addresses, port numbers, or
storage for further policy rules. It may also fail because of a
conflict with an established policy rule. In case of a conflict,
the first-come first-served mechanism is applied. Existing policy
rules remain unchanged and arriving new ones are rejected.
However, in case of a non-conflicting overlap of policy rules
(including identical policy rules), all policy rules are accepted.
The middlebox chooses a lifetime value that is greater than zero
and less than or equal to the minimum of the requested value and
the maximum lifetime specified by the middlebox at session
startup, i.e.,
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0 <= lt_granted <= MINIMUM(lt_requested, lt_maximum)
where
- lt_granted is the lifetime actually granted by the middlebox
- lt_requested is the lifetime the agent requested
- lt_maximum is the maximum lifetime specified at session
setup
In each case of failure, an appropriate failure reply is
generated. The policy reserve rule that is referenced in the PER
transaction is not affected in case of a failure within the PER
transaction -- i.e., the policy reserve rule remains.
After a new policy enable rule is successfully established and the
reply message has been sent to the requesting agent, the middlebox
checks whether there are other authenticated agents participating
in open sessions that can access the new policy rule. If the
middlebox finds one or more of these agents, then it sends a REN
message reporting the new policy rule to each of them.
2.3.10. Policy Rule Lifetime Change (RLC)
transaction-name: policy rule lifetime change
transaction-type: configuration
transaction-compliance: mandatory
request-parameters:
- request identifier: An agent-unique identifier for matching
corresponding request and reply at the agent.
- policy rule identifier: Identifying the policy rule for which
the lifetime is requested to be changed. This may identify
either a policy reserve rule or a policy enable rule.
- policy rule lifetime: The new lifetime proposal for the policy
rule.
reply-parameters (success):
- request identifier: An identifier matching the identifier of the
request.
- policy rule lifetime: The remaining policy rule lifetime granted
by the middlebox.
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failure reason:
- agent not authorized for this transaction
- agent not authorized to change lifetime of this policy
rule
- no such policy rule
- lifetime cannot be extended
notification message type: Policy Rule Event Notification (REN)
semantics:
The agent can use this transaction type to request the extension
of an established policy rule's lifetime, the shortening of the
lifetime, or policy rule termination. Policy rule termination is
requested by suggesting a new policy rule lifetime of zero.
The middlebox first checks whether the specified policy rule
exists and whether the agent is authorized to access this policy
rule. If one of the checks fails, an appropriate failure reply is
generated. If the requested lifetime is longer than the current
one, the middlebox also checks whether the lifetime of the policy
rule may be extended and generates an appropriate failure message
if it may not.
A failure reply implies that the new lifetime was not accepted,
and the policy rule remains unchanged. A success reply is
generated by the middlebox if the lifetime of the policy rule was
changed in any way.
The success reply contains the new lifetime of the policy rule.
The middlebox chooses a lifetime value that is greater than zero
and less than or equal to the minimum of the requested value and
the maximum lifetime specified by the middlebox at session
startup, i.e.,
0 <= lt_granted <= MINIMUM(lt_requested, lt_maximum)
whereas
- lt_granted is the lifetime actually granted by the middlebox
- lt_requested is the lifetime the agent requested
- lt_maximum is the maximum lifetime specified at session
setup
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After sending a success reply with a lifetime of zero, the
middlebox will consider the policy rule non-existent. Any further
transaction on this policy rule results in a negative reply,
indicating that this policy rule does not exist anymore.
Note that policy rule lifetime may also be changed by the Group
Lifetime Change (GLC) transaction, if applied to the group of
which the policy rule is a member.
After the remaining policy rule lifetime was successfully changed
and the reply message has been sent to the requesting agent, the
middlebox checks whether there are other authenticated agents
participating in open sessions that can access the policy rule.
If the middlebox finds one or more of these agents, then it sends
a REN message reporting the new remaining policy rule lifetime to
each of them.
2.3.11. Policy Rule List (PRL)
transaction-name: policy rule list
transaction-type: monitoring
transaction-compliance: mandatory
request-parameters:
- request identifier: An agent-unique identifier for matching
corresponding request and reply at the agent.
reply-parameters (success):
- request identifier: An identifier matching the identifier of the
request.
- policy list: List of policy rule identifiers of all policy rules
that the agent can access.
failure reason:
- transaction not supported
- agent not authorized for this transaction
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semantics:
The agent can use this transaction type to list all policies that
it can access. Usually, the agent has this information already,
but in special cases (for example, after an agent fail-over) or
for special agents (for example, an administrating agent that can
access all policies) this transaction can be helpful.
The middlebox first checks whether the agent is authorized to
request this transaction. If the check fails, an appropriate
failure reply is generated. Otherwise a list of all policies the
agent can access is returned indicating the identifier and the
owner of each policy.
This transaction does not have any effect on the policy rule
state.
2.3.12. Policy Rule Status (PRS)
transaction-name: policy rule status
transaction-type: monitoring
transaction-compliance: mandatory
request-parameters:
- request identifier: An agent-unique identifier for matching
corresponding request and reply at the agent.
- policy rule identifier: The middlebox-unique policy rule
identifier.
reply-parameters (success):
- request identifier: An identifier matching the identifier of the
request.
- policy rule owner: An identifier of the agent owning this policy
rule.
- group identifier: A reference to the group of which the policy
rule is a member.
- policy rule action: This parameter has either the value
'reserve' or the value 'enable'.
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- transport protocol: Identifies the protocol for which a
reservation is requested; see section 2.3.5.
- port range: The number of consecutive port numbers; see section
2.3.5.
- direction: The direction of the communication enabled by the
middlebox. Applicable only to policy enable rules.
- internal IP address version: The version of the internal IP
address (IP version of A0 in Figure 3).
- external IP address version: The version of the external IP
address (IP version of A3 in Figure 3).
- internal IP address: The IP address of the internal
communication endpoint (A0 in Figure 3); see section 2.3.5.
- internal port number: The port number of the internal
communication endpoint (A0 in Figure 3); see section 2.3.5.
- external IP address: The IP address of the external
communication endpoint (A3 in Figure 3); see section 2.3.5.
- external port number: The port number of the external
communication endpoint (A3 in Figure 3); see section 2.3.5.
- inside interface (optional): The inside interface at the
middlebox; see section 2.3.7.
- inside IP address: The internal IP address provided at the
inside of the NAT (A1 in Figure 3); see section 2.3.5.
- inside port number: The internal port number provided at the
inside of the NAT (A1 in Figure 3); see section 2.3.5.
- outside interface (optional): The outside interface at the
middlebox; see section 2.3.7.
- outside IP address: The external IP address provided at the
outside of the NAT (A2 in Figure 3); see section 2.3.5.
- outside port number: The external port number provided at the
outside of the NAT (A2 in Figure 3); see section 2.3.5.
- port parity: The parity of the allocated ports.
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- service: The selected service in the case of mixed traditional
and twice-NAT middlebox (see section 2.3.8).
- policy rule lifetime: The remaining lifetime of the policy rule.
failure reason:
- transaction not supported
- agent not authorized for this transaction
- no such policy rule
- agent not authorized to access this policy rule
semantics:
The agent can use this transaction type to list all properties of
a policy rule. Usually, the agent has this information already,
but in special cases (for example, after an agent fail-over) or
for special agents (for example, an administrating agent that can
access all policy rules) this transaction can be helpful.
The middlebox first checks whether the specified policy rule
exists and whether the agent is authorized to access this group.
If one of the checks fails, an appropriate failure reply is
generated. Otherwise all properties of the policy rule are
returned to the agent. Some of the returned parameters may be
irrelevant, depending on the policy rule action ('reserve' or
'enable') and depending on other parameters -- for example, the
protocol identifier.
This transaction does not have any effect on the policy rule
state.
2.3.13. Asynchronous Policy Rule Event (ARE)
transaction-name: asynchronous policy rule event
transaction-type: notification
transaction-compliance: mandatory
notification message type: Policy Rule Event Notification (REN)
semantics:
The middlebox may decide at any point in time to terminate a
policy rule. This transaction is triggered most frequently by
lifetime expiration of the policy rule. Among other events that
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may cause this transaction are changes in the policy rule decision
point.
The middlebox sends an REN message to all agents that participate
in an open session with the middlebox and that are authorized to
access the policy rule. The notification is sent to the agents
before the middlebox changes the policy rule's lifetime. The
change of lifetime may be triggered by any other authorized agent
and results in shortening (lt_new < lt_existing), extending
(lt_new > lt_existing), or terminating the policy rule
(lt_new = 0).
The ARE transaction corresponds to the REN message handling described
in section 2.3.4 for multiple agents.
2.3.14. Policy Rule State Machine
The state machine for the policy rule transactions is shown in Figure
4 with all possible state transitions. The used transaction
abbreviations may be found in the headings of the particular
transaction section.
PRR/success +---------------+
+-----------------+ PRID UNUSED |0)/ | PER/success +---------------+ |
success +---------------->| ENABLED +--+
+-+-------------+
| ^
lt = lifetime +-----------+
RLC(lt>0)/success
Figure 4: Policy Rule State Machine
This state machine exists per policy rule identifier (PRID).
Initially all policy rules are in state PRID UNUSED, which means that
the policy rule does not exist or is not active. After returning to
state PRID UNUSED, the policy rule identifier is no longer bound to
an existing policy rule and may be reused by the middlebox.
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A successful PRR transaction causes a transition from the initial
state PRID UNUSED to the state RESERVED, where an address reservation
is established. From there, state ENABLED can be entered by a PER
transaction. This transaction can also be used for entering state
ENABLED directly from state PRID UNUSED without a reservation. In
state ENABLED the requested communication between the internal and
the external endpoint is enabled.
The states RESERVED and ENABLED can be maintained by successful RLC
transactions with a requested lifetime greater than 0. Transitions
from both of these states back to state PRID UNUSED can be caused by
an ARE transaction or by a successful RLC transaction with a lifetime
parameter of 0.
A failed request transactions does not change state at the middlebox.
Note that transitions initiated by RLC transactions may also be
initiated by GLC transactions.
2.4. Policy Rule Group Transactions
This section describes the semantics for transactions on groups of
policy rules. These transactions are specified as follows:
- Group Lifetime Change (GLC)
- Group List (GL)
- Group Status (GS)
All are request transactions initiated by the agent. GLC is a
convenience transaction. GL and GS are monitoring transactions that
do not have any effect on the group state machine.
2.4.1. Overview
A policy rule group has only one attribute: the list of its members.
All member policies of a single group must be owned by the same
authenticated agent. Therefore, an implicit property of a group is
its owner, which is the owner of the member policy rules.
A group is implicitly created when its first member policy rule is
established. A group is implicitly terminated when the last
remaining member policy rule is terminated. Consequently, the
lifetime of a group is the maximum of the lifetimes of all member
policy rules.
A group has a middlebox-unique identifier.
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Group transactions are declared as 'optional' by their respective
compliance entry in section 3. However, they provide some
functionalities, such as convenience for the agent in sending only
one request instead of several, that is not available if only
mandatory transactions are available.
The Group Lifetime Change (GLC) transaction is equivalent to
simultaneously performed Policy Rule Lifetime Change (RLC)
transactions on all members of the group. The result of a successful
GLC transaction is that all member policy rules have the same
lifetime. As with the RLC transaction, the GLC transaction can be
used to delete all member policy rules by requesting a lifetime of
zero.
The monitoring transactions Group List (GL) and Group Status (GS) can
be used by the agent to explore the state of the middlebox and to
explore its access rights. The GL transaction lists all groups that
the agent may access, including groups owned by other agents. The GS
transaction reports the status on an individual group and lists all
policy rules of this group by their policy rule identifiers. The
agent can explore the state of the individual policy rules by using
the policy rule identifiers in a policy rule status (PRS) transaction
(see section 2.3.12).
The GL and GS transactions are particularly helpful in case of an
agent fail-over. The agent taking over the role of a failed one can
use these transactions retrieve whichever policies have been
established by the failed agent.
Notifications on group events are generated analogously to policy
rule events. To notify agents about group events, the Policy Rule
Group Event Notification (GEN) message type is used. GEN messages
contain an agent-unique notification identifier, the policy rule
group identifier, and the remaining lifetime of the group.
2.4.2. Group Lifetime Change (GLC)
transaction-name: group lifetime change
transaction-type: convenience
transaction-compliance: optional
request-parameters:
- request identifier: An agent-unique identifier for matching
corresponding request and reply at the agent.
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- group identifier: A reference to the group for which the
lifetime is requested to be changed.
- group lifetime: The new lifetime proposal for the group.
reply-parameters (success):
- request identifier: An identifier matching the identifier of the
request.
- group lifetime: The group lifetime granted by the middlebox.
failure reason:
- transaction not supported
- agent not authorized for this transaction
- agent not authorized to change lifetime of this group
- no such group
- lifetime cannot be extended
notification message type: Policy Rule Group Event Notification (GEN)
semantics:
The agent can use this transaction type to request an extension of
the lifetime of all members of a policy rule group, to request
shortening the lifetime of all members, or to request termination
of all member policies (which implies termination of the group).
Termination is requested by suggesting a new group lifetime of
zero.
The middlebox first checks whether the specified group exists and
whether the agent is authorized to access this group. If one of
the checks fails, an appropriate failure reply is generated. If
the requested lifetime is longer than the current one, the
middlebox also checks whether the lifetime of the group may be
extended and generates an appropriate failure message if it may
not.
A failure reply implies that the lifetime of the group remains
unchanged. A success reply is generated by the middlebox if the
lifetime of the group was changed in any way.
The success reply contains the new common lifetime of all member
policy rules of the group. The middlebox chooses the new lifetime
less than or equal to the minimum of the requested lifetime and
the maximum lifetime that the middlebox specified at session setup
along with its other capabilities, i.e.,
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0 <= lt_granted <= MINIMUM(lt_requested, lt_maximum)
where
- lt_granted is the lifetime actually granted by the middlebox
- lt_requested is the lifetime the agent requested
- lt_maximum is the maximum lifetime specified at session
setup
After sending a success reply with a lifetime of zero, the
middlebox will terminate the member policy rules without any
further notification to the agent, and will consider the group and
all of its members non-existent. Any further transaction on this
policy rule group or on any of its members results in a negative
reply, indicating that this group or policy rule, respectively,
does not exist anymore.
After the remaining policy rule group lifetime is successfully
changed and the reply message has been sent to the requesting
agent, the middlebox checks whether there are other authenticated
agents participating in open sessions that can access the policy
rule group. If the middlebox finds one or more of these agents,
it sends a GEN message reporting the new remaining policy rule
group lifetime to each of them.
2.4.3. Group List (GL)
transaction-name: group list
transaction-type: monitoring
transaction-compliance: optional
request-parameters:
- request identifier: An agent-unique identifier for matching
corresponding request and reply at the agent.
reply-parameters (success):
- request identifier: An identifier matching the identifier of the
request.
- group list: List of all groups that the agent can access. For
each listed group, the identifier and the owner are indicated.
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failure reason:
- transaction not supported
- agent not authorized for this transaction
semantics:
The agent can use this transaction type to list all groups that it
can access. Usually, the agent has this information already, but
in special cases (for example, after an agent fail-over) or for
special agents (for example, an administrating agent that can
access all groups) this transaction can be helpful.
The middlebox first checks whether the agent is authorized to
request this transaction. If the check fails, an appropriate
failure reply is generated. Otherwise a list of all groups the
agent can access is returned indicating the identifier and the
owner of each group.
This transaction does not have any effect on the group state.
2.4.4. Group Status (GS)
transaction-name: group status
transaction-type: monitoring
transaction-compliance: optional
request-parameters:
- request identifier: An agent-unique identifier for matching
corresponding request and reply at the agent.
- group identifier: A reference to the group for which status
information is requested.
reply-parameters (success):
- request identifier: An identifier matching the identifier of the
request.
- group owner: An identifier of the agent owning this policy rule
group.
- group lifetime: The remaining lifetime of the group. This is
the maximum of the remaining lifetime of all members, policy
rules.
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- member list: List of all policy rules that are members of the
group. The policy rules are specified by their middlebox-unique
policy rule identifier.
failure reason:
- transaction not supported
- agent not authorized for this transaction
- no such group
- agent not authorized to list members of this group
semantics:
The agent can use this transaction type to list all member policy
rules of a group. Usually, the agent has this information
already, but in special cases (for example, after an agent fail-
over) or for special agents (for example, an administrating agent
that can access all groups) this transaction can be helpful.
The middlebox first checks whether the specified group exists and
whether the agent is authorized to access this group. If one of
the checks fails, an appropriate failure reply is generated.
Otherwise a list of all group members is returned indicating the
identifier of each group.
This transaction does not have any effect on the group state.
3. Conformance Statements
A protocol definition complies with the semantics defined in section
2 if the protocol specification includes all specified transactions
with all their mandatory parameters. However, concrete
implementations of the protocol may support only some of the optional
transactions, not all of them. Which transactions are required for
compliance is different for agent and middlebox.
This section contains conformance statements for MIDCOM protocol
implementations related to the semantics. Conformance is specified
differently for agents and middleboxes. These conformance statements
will probably be extended by a concrete protocol specification.
However, such an extension is expected to extend the statements below
in such a way that all of them still hold.
The following list shows the transaction-compliance property of all
transactions as specified in the previous section:
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- Session Control Transactions
- Session Establishment (SE) mandatory
- Session Termination (ST) mandatory
- Asynchronous Session Termination (AST) mandatory
- Policy Rule Transactions
- Policy Reserve Rule (PRR) mandatory
- Policy Enable Rule (PER) mandatory
- Policy Rule Lifetime Change (RLC) mandatory
- Policy Rule List (PRL) mandatory
- Policy Rule Status (PRS) mandatory
- Asynchronous Policy Rule Event (ARE) mandatory
- Policy Rule Group Transactions
- Group Lifetime Change (GLC) optional
- Group List (GL) optional
- Group Status (GS) optional
3.1. General Implementation Conformance
A compliant implementation of a MIDCOM protocol must support all
mandatory transactions.
A compliant implementation of a MIDCOM protocol may support none,
one, or more of the following transactions: GLC, GL, GS.
A compliant implementation may extend the protocol semantics by
further transactions.
A compliant implementation of a MIDCOM protocol must support all
mandatory parameters of each transaction concerning the information
contained. The set of parameters can be redefined per transaction as
long as the contained information is maintained.
A compliant implementation of a MIDCOM protocol may support the use
of interface-specific policy rules. Either both or neither of the
optional inside and outside interface parameters in PRR, PER, and PRS
must be included when interface-specific policy rules are supported.
A compliant implementation may extend the list of parameters of
transactions.
A compliant implementation may replace a single transaction by a set
of more fine-grained transactions. In such a case, it must be
ensured that requirement 2.1.4 (deterministic behavior) and
requirement 2.1.5 (known and stable state) of [MDC-REQ] are still
met. When a single transaction is replaced by a set of multiple
fine-grained transactions, this set must be equivalent to a single
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transaction. Furthermore, this set of transactions must further meet
the atomicity requirement stated in section 2.1.3.
3.2. Middlebox Conformance
A middlebox implementation of a MIDCOM protocol supports a request
transaction if it is able to receive and process all possible correct
message instances of the particular request transaction and if it
generates a correct reply for any correct request it receives.
A middlebox implementation of a MIDCOM protocol supports an
asynchronous transaction if it is able to generate the corresponding
notification message properly.
A compliant middlebox implementation of a MIDCOM protocol must inform
the agent about the list of supported transactions within the SE
transaction.
3.3. Agent Conformance
An agent implementation of a MIDCOM protocol supports a request
transaction if it can generate the corresponding request message
properly and if it can receive and process all possible correct
replies to the particular request.
An agent implementation of a MIDCOM protocol supports an asynchronous
transaction if it can receive and process all possible correct
message instances of the particular transaction.
A compliant agent implementation of a MIDCOM protocol must not use
any optional transaction that is not supported by the middlebox. The
middlebox informs the agent about the list of supported transactions
within the SE transaction.
4. Transaction Usage Examples
This section gives two usage examples of the transactions specified
in Section 2. The first shows how an agent can explore all policy
rules and policy rule groups that it may access at a middlebox. The
second example shows the configuration of a middlebox in combination
with the setup of a voice over IP session with the Session Initiation
Protocol (SIP) [RFC3261].
4.1. Exploring Policy Rules and Policy Rule Groups
This example assumes an already established session. It shows how an
agent can find out
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- which groups it may access and who owns these groups,
- the status and member list of all accessible groups, and
- the status and properties of all accessible policy rules.
If there is just a single session, these actions are not needed,
because the middlebox informs the agent about each state transition
of any policy rule or policy rule group. However, after the
disruption of a session or after an intentional session termination,
the agent might want to re-establish the session and explore which of
the groups and policy rules it established are still in place.
Also, an agent system may fail and another one may take over. Then
the new agent system needs to find out what has already been
configured by the failing system and what still needs to be done.
A third situation where exploring policy rules and groups is useful
is the case of an agent with 'administrator' authorization. This
agent may access and modify any policy rule or group created by any
other agent.
All agents will probably start their exploration with the Group List
(GL) transaction, as shown in Figure 5. On this request, the
middlebox returns a list of pairs, each containing an agent
identifier and a group identifier (GID). The agent is informed which
of its own groups and which other agents' groups it may access.
agent middlebox
| GL |
|**********************************************>|
||
|
RFC 3989 MIDCOM Protocol Semantics February 2005
In the following, the agent explores these four policy rules. The
example assumes that the middlebox is a traditional NAPT. Figure 6
shows the exploration of the first policy rule. In reply to a Policy
Rule Status (PRS) transaction, the middlebox always returns the
following list of parameters:
- policy rule owner
- group identifier
- policy rule action (reserve or enable)
- protocol type
- port range
- direction
- internal IP address
- internal port number
- external address
- external port number
- middlebox inside IP address
- middlebox inside port number
- middlebox outside IP address
- middlebox outside port number
- IP address versions (not printed)
- middlebox service (not printed)
- inside and outside interface (optional, not printed)
agent middlebox
| PRS PID1 |
|**********************************************>|
|
RFC 3989 MIDCOM Protocol Semantics February 2005
Applying PRS to PID2 (Figure 7) shows that the second policy rule is
a policy enable rule for inbound UDP packets. The internal
destination is fixed concerning IP address, protocol, and port
number, but for the external source, the port number is wildcarded.
The outside IP address and port number of the middlebox are what the
external sender needs to use as destination in the original packet it
sends. At the middlebox, the destination address is replaced with
the internal address of the final receiver. During address
translation, the source IP address and the source port numbers of the
packets remain unchanged. This is indicated by the inside address,
which is identical to the external address.
agent middlebox
| PRS PID2 |
|**********************************************>|
||
|
RFC 3989 MIDCOM Protocol Semantics February 2005
agent middlebox
| PRS PID4 |
|**********************************************>|
|| Middlebox || SIP User Agent |
| Agent A || NAPT || B@example.org |
+----------+ +-----------+ +----------------+
SIP Signaling
RTP Traffic
++++ MIDCOM protocol
Figure 10: Example of a SIP Scenario
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For the sequence charts below, we make these assumptions:
- The NAPT is statically configured to forward SIP signaling from
the outside to the SIP proxy server -- i.e., traffic to the
NAPT's external IP address and port 5060 is forwarded to the
internal SIP proxy.
- The SIP user agent A, located inside the private network, is
registered at the SIP proxy with its private IP address.
- User A knows the general SIP URL of user B. The URL is
B@example.org. However, the concrete URL of the SIP User Agent
B, which user B currently uses, is not known.
- The RTP paths are configured, but not the RTCP paths.
- The middlebox and the SIP server share an established MIDCOM
session.
- Some parameters are omitted, such as the request identifier
(RID).
Furthermore, the following abbreviations are used:
- IP_AI: Internal IP address of user agent A
- P_AI: Internal port number of user agent A to receive RTP data
- P_AE: External mapped port number of user agent A
- IP_AE: External IP address of the middlebox
- IP_B: IP address of user agent B
- P_B: Port number of user agent B to receive RTP data
- GID: Group identifier
- PID: Policy rule identifier
The abbreviations of the MIDCOM transactions can be found in the
particular section headings.
In our example, user A tries to call user B. The user agent A sends
an INVITE SIP message to the SIP proxy server (see Figure 10). The
SDP part of the particular SIP message relevant for the middlebox
configuration is shown in the sequence chart as follows:
SDP: m=..P_AI..
c=IP_AI
where the m tag is the media tag that contains the receiving UDP port
number, and the c tag contains the IP address of the terminal
receiving the media stream.
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The INVITE message forwarded to user agent B must contain a public IP
address and a port number to which user agent B can send its RTP
media stream. The SIP proxy requests a policy enable rule at the
middlebox with a PER request with the wildcarded IP address and port
number of user agent B. As neither the IP address nor port numbers
of user agent B are known at this point, the address of user agent B
must be wildcarded. The wildcarded IP address and port number
enables the 'early media' capability but results in some insecurity,
as any outside host can reach user agent A on the enabled port number
through the middlebox.
User Agent SIP Middlebox User Agent
A Proxy NAPT B
| | | |
| INIVTE | | |
| B@example.org | | |
| SDP:m=..P_AI.. | | |
| c=IP_AI | | |
|--------------->| | |
| | | |
| | PER PID1 UDP 1 EVEN IN | |
| | IP_AI P_AI ANY ANY 300s | |
| |*****************************>| |
| |
RFC 3989 MIDCOM Protocol Semantics February 2005
By using the PRR request, the SIP proxy requests an outside IP
address and port number (see Figure 12) without already establishing
a NAT binding or pin hole. The PRR request contains the service
parameter 'tw' -- i.e., the MIDCOM agent chooses the default value.
In this configuration, with NAPT and without a twice NAT, only an
outside address is reserved. In the SDP payload of the INVITE
message, the SIP proxy server replaces the IP address and port number
of user agent A with the reserved IP address and port from PRR reply
(see Figure 12). The SIP INVITE message is forwarded to user agent B
with a modified SDP body containing the outside address and port
number, to which user agent B will send its RTP media stream.
User Agent SIP Middlebox User Agent
A Proxy NAPT B
| | | |
...PER in Figure 11 has failed, continuing with PRR ...
| | | |
| |PRR tw v4 v4 A UDP 1 EVEN 300s| |
| |*****************************>| |
| ||
| |
RFC 3989 MIDCOM Protocol Semantics February 2005
members of the same group. After having enabled both UDP/RTP
streams, the SIP proxy can forward the '200 OK' SIP message to user
agent A to indicate that the telephone call can start.
User Agent SIP Middlebox User Agent
A Proxy NAPT B
| | | |
| | PER PID1 UDP 1 SAME IN | |
| | IP_AI P_AI IP_B ANY 300s | |
| |*****************************>| |
| || |
| |
RFC 3989 MIDCOM Protocol Semantics February 2005
User Agent SIP Middlebox User Agent
A Proxy NAPT B
| | | |
| BYE | BYE |
||-------------------------------------------->|
| | | |
| | GLC GID1 0s | |
| |*****************************>| |
| |
RFC 3989 MIDCOM Protocol Semantics February 2005
5.1.2. Agent Connects to Multiple Middleboxes
As specified in section 2.2, the MIDCOM protocol allows the agent to
communicate with more than one middlebox simultaneously. The
selection of a mechanism for separating different sessions is left to
the concrete protocol definition. It must provide a clear mapping of
protocol messages to open sessions. Then requirement 2.1.2 is met.
5.1.3. Multiple Agents Connect to same Middlebox
As specified in section 2.2, the MIDCOM protocol allows the middlebox
to communicate with more than one agent simultaneously. The
selection of a mechanism for separating different sessions is left to
the concrete protocol definition. It must provide a clear mapping of
protocol messages to open sessions. Then requirement 2.1.3 is met.
5.1.4. Deterministic Behavior
Section 2.1.2 states that the processing of a request of an agent may
not be interrupted by any request of the same or another agent. This
provides atomicity among request transactions and avoids race
conditions resulting in unpredictable behavior by the middlebox.
The behavior of the middlebox can only be predictable in the view of
its administrators. In the view of an agent, the middlebox behavior
is unpredictable, as the administrator can, for example, modify the
authorization of the agent at any time without the agent being able
to observe this change. Consequently, the behavior of the middlebox
is not necessarily deterministic from the point of view of any agent.
As predictability of the middlebox behavior is given for its
administrator, requirement 2.1.4 is met.
5.1.5. Known and Stable State
Section 2.1 states that request transactions are atomic with respect
to each other and from the point of view of an agent. All
transactions are clearly defined as state transitions that either
leave the current stable, well-defined state and enter a new stable,
well-defined one or that remain in the current stable, well-defined
state. Section 2.1 clearly demands that intermediate states are not
stable and are not reported to any agent.
Furthermore, for each state transition a message is sent to the
corresponding agent, either a reply or a notification. The agent can
uniquely map each reply to one of the requests that it sent to the
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middlebox, because agent-unique request identifiers are used for this
purpose. Notifications are self-explanatory by their definition.
Furthermore, the Group List transaction (section 2.4.3), the Group
Status transaction (section 2.4.4), the Policy Rule List transaction
(section 2.3.11), and the Policy Rule Status transaction (section
2.3.12) allow the agent at any time during a session to retrieve
information about
- all policy rule groups it may access,
- the status and member policy rules of all accessible groups,
- all policy rules it may access, and
- the status of all accessible policy rules.
Therefore, the agent is precisely informed about the state of the
middlebox (as far as the services requested by the agent are
affected), and requirement 2.1.5 is met.
5.1.6. Status Report
As argued in the previous section, the middlebox unambiguously
informs the agent about every state transition related to any of the
services requested by the agent. Also, at any time the agent can
retrieve full status information about all accessible policy rules
and policy rule groups. Thus, requirement 2.1.6 is met.
5.1.7. Unsolicited Messages (Asynchronous Notifications)
The semantics includes asynchronous notifications messages from the
middlebox to the agent, including the Session Termination
Notification message, the Policy Rule Event Notification (REN)
message, and the Group Event Notification (GEN) message (see section
2.1.2). These notifications report every change of state of policy
rules or policy rule groups that was not explicitly requested by the
agent. Thus, requirement 2.1.7 is met by the semantics specified
above.
5.1.8. Mutual Authentication
As specified in section 2.2.1, the semantics requires mutual
authentication of agent and middlebox, by using either two subsequent
Session Establishment transactions or mutual authentication provided
on a lower protocol layer. Thus, requirement 2.1.8 is met.
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5.1.9. Session Termination by Any Party
The semantics specification states in section 2.2.2 that the agent
may request session termination by generating the Session Termination
request and that the middlebox may not reject this request. In turn,
section 2.2.3 states that the middlebox may send the Asynchronous
Session Termination notification at any time and then terminate the
session. Thus, requirement 2.1.9 is met.
5.1.10. Request Result
Section 2.1 states that each request of an agent is followed by a
reply of the middlebox indicating either success or failure. Thus,
requirement 2.2.10 is met.
5.1.11. Version Interworking
Section 2.2.1 states that the agent needs to specify the protocol
version number that it will use during the session. The middlebox
may accept this and act according to this protocol version or may
reject the session if it does not support this version. If the
session setup is rejected, the agent may try again with another
version. Thus, requirement 2.2.11 is met.
5.1.12. Deterministic Handling of Overlapping Rules
The only policy rule actions specified are 'reserve' and 'enable'.
For firewalls, overlapping enable actions or reserve actions do not
create any conflict, so a firewall will always accept overlapping
rules as specified in section 2.3.2 (assuming the required
authorization is given).
For NATs, reserve and enable may conflict. If a conflicting request
arrives, it is rejected, as stated in section 2.3.2. If an
overlapping request arrives that does not conflict with those it
overlaps, it is accepted (assuming the required authorization is
given).
Therefore, the behavior of the middlebox in the presence of
overlapping rules can be predicted deterministically, and requirement
2.1.12 is met.
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5.2. Protocol Semantics Requirements
5.2.1. Extensible Syntax and Semantics
Requirement 2.2.1 explicitly requests extensibility of protocol
syntax. This needs to be addressed by the concrete protocol
definition. The semantics specification is extensible anyway,
because new transactions may be added.
5.2.2. Policy Rules for Different Types of Middleboxes
Section 2.3 explains that the semantics uses identical transactions
for all middlebox types and that the same policy rule can be applied
to all of them. Thus, requirement 2.2.2 is met.
5.2.3. Ruleset Groups
The semantics explicitly supports grouping of policy rules and
transactions on policy rule groups, as described in section 2.4. The
group transactions can be used for lifetime extension and termination
of all policy rules that are members of the particular group. Thus,
requirement 2.2.3 is met.
5.2.4. Policy Rule Lifetime Extension
The semantics includes a transaction for explicit lifetime extension
of policy rules, as described in section 2.3.3. Thus, requirement
2.2.4 is met.
5.2.5. Robust Failure Modes
The state transitions at the middlebox are clearly specified and
communicated to the agent. There is no intermediate state reached by
a partial processing of a request. All requests are always processed
completely, either successfully or unsuccessfully. All request
transactions include a list of failure reasons. These failure
reasons cover indication of invalid parameters where applicable. In
case of failure, one of the specified reasons is returned from the
middlebox to the agent. Thus, requirement 2.2.5 is met.
5.2.6. Failure Reasons
The semantics includes a failure reason parameter in each failure
reply. Thus, requirement 2.2.6 is met.
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5.2.7. Multiple Agents Manipulating Same Policy Rule
As specified in sections 2.3 and 2.4, each installed policy rule and
policy rule group has an owner, which is the authenticated agent that
created the policy rule or group, respectively. The authenticated
identity is input to authorize access to policy rules and groups.
If the middlebox is sufficiently configurable, its administrator can
configure it so that one authenticated agent is authorized to access
and modify policy rules and groups owned by another agent. Because
specified semantics does not preclude this, it meets requirement
2.2.7.
5.2.8. Carrying Filtering Rules
The Policy Enable Rule transaction specified in section 2.3.8 can
carry 5-tuple filtering rules. This meets requirement 2.2.8.
5.2.9. Parity of Port Numbers
As specified in section 2.3.6, the agent is able to request keeping
the port parity when reserving port numbers with the PRR transaction
(see section 2.3.8) and when establishing address bindings with the
PER transaction (see section 2.3.9). Thus requirement 2.2.9 is met.
5.2.10. Consecutive Range of Port Numbers
As specified in section 2.3.6, the agent is able to request a
consecutive range of port numbers when reserving port numbers with
the PRR transaction (see section 2.3.8) and when establishing address
bindings or pinholes with the PER transaction (see section 2.3.9).
Thus requirement 2.2.10 is met.
5.2.11. Contradicting Overlapping Policy Rules
Requirement 2.2.11 is based on the assumption that contradictory
policy rule actions, such as 'enable'/'allow' and
'disable'/'disallows' are supported. In conformance with decisions
made by the working group after finalizing the requirements document,
this requirement is not met by the semantics because no
'disable'/'disallow' action is supported.
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5.3. Security Requirements
5.3.1. Authentication, Confidentiality, Integrity
The semantics definition supports mutual authentication of agent and
middlebox in the Session Establishment transaction (section 2.2.1).
The use of an underlying protocol such as TLS or IPsec is mandatory.
Thus, requirement 2.3.1 is met.
5.3.2. Optional Confidentiality of Control Messages
The use of IPsec or TLS allows agent and middlebox to use an
encryption method (including no encryption). Thus, requirement 2.3.2
is met.
5.3.3. Operation across Untrusted Domains
Operation across untrusted domains is supported by mutual
authentication and by the use of TLS or IPsec protection. Thus,
requirement 2.3.3 is met.
5.3.4. Mitigate Replay Attacks
The specified semantics mitigates replay attacks and meets
requirement 2.3.4 by requiring mutual authentication of agent and
middlebox, and by mandating the use of TLS or IPsec protection.
Further mitigation can be provided as part of a concrete MIDCOM
protocol definition -- for example, by requiring consecutively
increasing numbers for request identifiers.
6. Security Considerations
The interaction between a middlebox and an agent (see [MDC-FRM]) is a
very sensitive point with respect to security. The configuration of
policy rules from a middlebox-external entity appears to contradict
the nature of a middlebox. Therefore, effective means have to be
used to ensure
- mutual authentication between agent and middlebox,
- authorization,
- message integrity, and
- message confidentiality.
The semantics defines a mechanism to ensure mutual authentication
between agent and middlebox (see section 2.2.1). In combination with
the authentication, the middlebox is able to decide whether an agent
is authorized to request an action at the middlebox. The semantics
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relies on underlying protocols, such as TLS or IPsec, to maintain
message integrity and confidentiality of the transferred data between
both entities.
For the TLS and IPsec use, both sides must use securely configured
credentials for authentication and authorization.
The configuration of policy rules with wildcarded IP addresses and
port numbers results in certain risks, such as opening overly
wildcarded policy rules. An excessively wildcarded policy rule would
be A0 and A3 with IP address set to 'any' IP address, for instance.
This type of pinhole would render the middlebox, in the sense of
security, useless, as any packet could traverse the middlebox without
further checking. The local policy of the middlebox should reject
such policy rule enable requests.
A reasonable default configuration for wildcarding would be that only
one port number may be wildcarded and all IP addresses must be set
without wildcarding. However, there are some cases where security
needs to be balanced with functionality.
The example described in section 4.2 shows how SIP-signaled calls can
be served in a secure way without wildcarding IP addresses. But some
SIP-signaled applications make use of early media (see section 5.5 of
[RFC3398]). To receive early media, the middleboxes need to be
configured before the second participant in a session is known. As
it is not known, the IP address of the second participant needs to be
wildcarded.
In such cases and in several similar ones, there is a security policy
decision to be made by the middlebox operator. The operator can
configure the middlebox so that it supports more functionality, for
example, by allowing wildcarded IP addresses, or so that network
operation is more secure, for example, by disallowing wildcarded IP
addresses.
7. IAB Considerations on UNSAF
UNilateral Self-Address Fixing (UNSAF) is described in [RFC3424] as a
process at originating endpoints that attempt to determine or fix the
address (and port) by which they are known to another endpoint.
UNSAF proposals, such as STUN [RFC3489] are considered as a general
class of workarounds for NAT traversal and as solutions for scenarios
with no middlebox communication (MIDCOM).
This document describes the protocol semantics for such a middlebox
communication (MIDCOM) solution. MIDCOM is not intended as a short-
term workaround, but more as a long-term solution for middlebox
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communication. In MIDCOM, endpoints are not involved in allocating,
maintaining, and deleting addresses and ports at the middlebox. The
full control of addresses and ports at the middlebox is located at
the MIDCOM server.
Therefore, this document addresses the UNSAF considerations in
[RFC3424] by proposing a long-term alternative solution.
8. Acknowledgements
We would like to thank all the people contributing to the semantics
discussion on the mailing list for a lot of valuable comments.
9. References
9.1. Normative References
[MDC-FRM] Srisuresh, P., Kuthan, J., Rosenberg, J., Molitor, A.,
and A. Rayhan, "Middlebox communication architecture and
framework", RFC 3303, August 2002.
[MDC-REQ] Swale, R., Mart, P., Sijben, P., Brim, S., and M. Shore,
"Middlebox Communications (midcom) Protocol
Requirements", RFC 3304, August 2002.
[NAT-TERM] Srisuresh, P. and M. Holdrege, "IP Network Address
Translator (NAT) Terminology and Considerations", RFC
2663, August 1999.
[NAT-TRAD] Srisuresh, P. and K. Egevang, "Traditional IP Network
Address Translator (Traditional NAT)", RFC 3022, January
2001.
9.2. Informative References
[RFC2246] Dierks, T. and C. Allen, "The TLS Protocol Version 1.0",
RFC 2246, January 1999.
[RFC2402] Kent, S. and R. Atkinson, "IP Authentication Header", RFC
2402, November 1998.
[RFC2406] Kent, S. and R. Atkinson, "IP Encapsulating Security
Payload (ESP)", RFC 2406, November 1998.
[RFC3198] Westerinen, A., Schnizlein, J., Strassner, J., Scherling,
M., Quinn, B., Herzog, S., Huynh, A., Carlson, M., Perry,
J., and S. Waldbusser, "Terminology for Policy-Based
Management", RFC 3198, November 2001.
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RFC 3989 MIDCOM Protocol Semantics February 2005
[RFC3261] Rosenberg, J., Schulzrinne, H., Camarillo, G., Johnston,
A., Peterson, J., Sparks, R., Handley, M., and E.
Schooler, "SIP: Session Initiation Protocol", RFC 3261,
June 2002.
[RFC3398] Camarillo, G., Roach, A., Peterson, J., and L. Ong,
"Integrated Services Digital Network (ISDN) User Part
(ISUP) to Session Initiation Protocol (SIP) Mapping", RFC
3398, December 2002.
[RFC3424] Daigle, L. and IAB, "IAB Considerations for UNilateral
Self-Address Fixing (UNSAF) Across Network Address
Translation", RFC 3424, November 2002.
[RFC3489] Rosenberg, J., Weinberger, J., Huitema, C., and R. Mahy,
"STUN - Simple Traversal of User Datagram Protocol (UDP)
Through Network Address Translators (NATs)", RFC 3489,
March 2003.
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RFC 3989 MIDCOM Protocol Semantics February 2005
Authors' Addresses
Martin Stiemerling
NEC Europe Ltd.
Network Laboratories
Kurfuersten-Anlage 36
69115 Heidelberg
Germany
Phone: +49 6221 90511-13
EMail: stiemerling@netlab.nec.de
Juergen Quittek
NEC Europe Ltd.
Network Laboratories
Kurfuersten-Anlage 36
69115 Heidelberg
Germany
Phone: +49 6221 90511-15
EMail: quittek@netlab.nec.de
Tom Taylor
Nortel
1852 Lorraine Ave.
Ottawa, Ontario
Canada K1H 6Z8
Phone: +1 613 763 1496
EMail: taylor@nortel.com
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RFC 3989 MIDCOM Protocol Semantics February 2005
Full Copyright Statement
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